friable permafrost

friendly ice

from the point of view of the submariner, an icey canopy containing many large ice skylights or other features which permit a submarine to surface; there must be more than ten such features per 37 kilometers (30 nautical miles) along the submarine's track.

front

in meteorology, generally, the interface or transition zone between two air masses of different density; since the temperature distribution is the most important regulator of the atmosphere density, a front almost invariably separates air masses of different temperature; when warmer air replaces the colder, it is a warm front, and vice-versa.

frost flowers

crystals of ice that form when water vapor becomes a solid (bypassing the liquid phase) and deposits itself on the sea ice surface; frost flowers roughen the surface and dramatically affect its electromagnetic signal.

Close-up view of frost flowers. (Photo courtesy of Don Perovich, U.S. Army Cold Regions Research and Engineering Laboratory.)

frozen ground

fusion

the phase transition of a substance passing from the solid to the liquid state, melting; in meteorology, fusion is understood to refer to the melting of ice, which, if the ice is pure and subjected to one standard atmosphere of pressure, takes place at the ice point of 0 degrees Celsius (32 degrees Fahrenheit).